What's causing the current drought?

Ingram and other paleoclimatologists have correlated several historic megadroughts with a shift in the surface temperature of the Pacific Ocean that occurs every 20 to 30 years—something called the Pacific Decadal Oscillation (PDO). The PDO is similar to an El Nino event except it lasts for decades—as its name implies—whereas an El Nino event lasts 6 to 18 months. Cool phases of the PDO result in less precipitation because cooler sea temperatures bump the jet stream north, which in turn pushes off storms that would otherwise provide rain and snow to California. Ingram says entire lakes dried up in California following a cool phase of the PDO several thousand years ago. Warm phases have been linked to numerous storms along the California coast.

"We have been in a fairly cold phase of PDO since the early 2000s," says Brian Fuchs, a climatologist at the National Drought Mitigation Center, "so the drought we are seeing now makes sense."

That said, scientists caution against pinning the current drought on the PDO alone. Certainly ocean temperatures, wind, and the weather pattern in the Pacific have contributed to the drought, says Nate Mantua, of NOAA's National Marine Fisheries Service in Santa Cruz, California, where the PDO pattern was first discovered and named. "But it's more nuanced than saying the PDO did this." After all, as its name suggests, the PDO is decades in the making.